Computational studies exploring the relationship between flame lift-off height and soot formation in diesel jets

Abstract

It has been suggested that fuel/air mixing upstream of the lift-off height influences the formation of soot in reacting diesel jets. Hence, greater lift-off height results in more mixing, resulting in less soot. In this work, computations of reacting diesel jets are carried out for a wide range of conditions by employing a RANS model in which an unsteady flamelet progress variable (UFPV) submodel is employed to represent turbulence/chemistry interactions. The conditions selected reflect changes in injection pressure, chamber temperature, oxygen concentration, ambient density, and orifice diameter. As reported in prior work, the UFPV model predicts the ignition delay and flame lift-off height within about 25% of reported measurements. Soot is modeled using a kinetic model in which hydrogen-abstraction followed by carbon-addition results in the formation of polycyclic aromatic hydrocarbons (PAHs) which act as precursors to soot. For all cases, except the cases with different orifice diameter and ambient density, the soot concentration decreases with increasing lift-off height when the lift-off height is appropriately normalized. Analysis of the entrained mass upstream of the lift-off height confirms that this correlation arises from variation in entrained mass.